Mixers of this type are used for folding, stirring, mixing, combining, blending, whipping, emulsifying, homogenizing and beating of substances retained in a bowl. Mixers of this kind, if they have one shaft, are also referred to as wand-type mixers; others have two parallel shafts. These mixers, and particularly the wand-type mixers, are supplied with mixing heads of different shapes which, respectively, can be attached to the shaft to carry out the desired operation; for example, operating heads intended for stirring of quite viscous substances, or combining of substances with each other, will have different shapes from those intended to cream substances, for example potatoes, or to beat eggwhites, or to whip cream. Other mixing heads may be used to chop substances, for example nuts, dried fruit and the like. These removable mixing heads usually have a metal sleeve forming a hub, adapted to be fitted over the operating shaft. A groove or notch may be provided in the hub sleeve, which is engaged by a carrier pin formed on the shaft, to transmit torque and rotary motion. The operating head, in operation of the appliance, of course should be securely seated on the end of the shaft and must not inadvertently become loose or detached from the shaft. To assure such positive engagement, the hub is frequently provided with an elastically deformable region, for example defined by a pair of axially extending slits. This region is slightly deflected outwardly when the hub is engaged with the shaft, in order to provide resilient engagement force for frictional engagement of the remainder of the sleeve with the shaft. The resilient region, thus, in effect forms a springy tab or flap, separated from the remainder of the hub by the two longitudinal slits or cuts.
The hub is usually chamfered around the outer edge in order to facilitate attachment of the hub on the shaft.
It has been found in use that this arrangement, which is simple and effective and entirely adequate for most operating conditions, yet requires careful attention upon joining the operating head to the shaft. If the user tries to engage the operating head while the hub is at an inclination with respect to the axis of the shaft, and then forces the operating head on the shaft, the springy tab will be bent outwardly. This bend may remain set, by plastic deformation. As a consequence, secure frictional engagement of the attached operating head on the operating shaft is no longer ensured. The operating head, thus, may inadvertently become detached from the shaft. If the springy engagement tab is deformed, it is possible to return it to its initial position, for example by bending it backwards with pliers in order to again provide for springy engagement of the hub with the shaft, and so that the springy tab can again function as a force-applying element. Re-bending the tab, however, requires some dexterity and mechanical ability on part of the user, which cannot always be expected. Repeated bending of the tab, also, weakens the joint between the tab and the remainder of the hub, which may lead to failure of the operating head, and permanent loss of springiness of the tab.